Means and Methods – In the Engineer’s Domain?

  

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13 
Restrictive Tolerances on Temporary Shaft Support 
Temporary shaft are just that, temporary. More than one contractor indicated that shafts tend to be 
fairly shallow and short-lived in-terms of how long the shaft will be used but often the contract 
documents frequently have vertical tolerances that relate to permanent shaft support. The 
impression given in contractor discussions was that specifications were taken from other projects 
with the shaft wall acting as temporary and permanent support and applied to shafts that are short-
lived and temporary only.
Overly Conservative Engineering Parameters for Temporary Shaft Support Design 
One contractor reported that in some cases owners issue pressure diagrams that are extremely 
conservative for temporary shaft support. The temporary shafts as a result are significantly 
overdesigned and result in far greater construction costs than necessary. It would be far better to 
mandate that the contractor hire a licensed engineer to design and seal the temporary shaft support 
and base that design on a geotechnical data report than to issue or baseline design parameters that 
are too conservative. The other related concern expressed was the use of the same pressure 
diagrams for a region rather than developing them for project specific conditions. The problem with 
the latter goes without saying.
Geotechnical Baselines not Measurable in the Field and Too Many Baselines 
 
The primary purpose of the baseline concept is to establish a framework for consideration of differing 
site conditions. As such, in theory every baseline should be established in large part to do its ability 
to be measured during construction. If not measureable, the variable or parameter adds little value 
to the project. Also noted by contractors is the trend to baseline any number of parameters 
regardless of their importance to tunnel construction. In reality, this trend actually increases owner 
risk as explanations can be developed to explain why such and such a parameter impacted means and 
methods. It is prudent to only baseline parameters that facilitate selection of means and methods 
and that are measureable in the field. Also, production is the primary barometer for profit on an 
underground project so parameters that impact the prediction of production should also be 
considered for baseline. As noted earlier, baselines are sometimes provided for the design of 
temporary support, which is reasonable, but these baselines also need to be realistic.